Ionic conductors that are also electronic insulators are called electrolytes; they may be a liquid or a solid. Electrolytes are used in a variety of electrochemical devices, including not only those that store electric power as chemical energy in a rechargeable battery or those that release chemical energy as electric power in a fuel cell, but also those that store electric power as static electric energy in an electric-double-layer capacitor. Electric power that is released from an electric-energy store, whether from a chemical or an electrostatic store, is clean energy. Chemical energy stored in a fuel that is released as the heat of combustion is a less efficient process, and combustion is also accompanied by the release of gases that pollute the air and contribute to global warming.
An electrochemical cell contains an electrolyte between two electrodes, an anode and a cathode. A liquid electrolyte requires use of a separator of the two electrodes that is permeable by the liquid electrolyte; the separator prevents electronic contact between the two electrodes within the cell. A solid electrolyte may serve as both an electrolyte and a separator. In a rechargeable battery, the anode is a reductant; in a fuel cell, the anode catalyzes the separation of a reductant fuel into its electronic and ionic components. In both types of cells, the ionic component of the chemical reaction between two electrodes is transported to the cathode inside the cell in the electrolyte, but the electrolyte forces the electronic component to go to the cathode via an external circuit as an electronic current I at a voltage V to provide electric power P=IV for performance of work. Since the ionic conductivity in the electrolyte is much smaller than the electronic conductivity in a good metal, battery cells and fuel cells are fabricated with large-area electrodes and a thin electrolyte; the active electrode materials are fabricated to make electronic contact with a metallic current collector for fast transport of electrons between the active electrode particles and the external circuit as well as ionic contact with the electrolyte that transports ions between the electrodes inside the cell.
Solid electrolytes with a large dielectric constant may also be used in electronic devices as separators of liquid or gaseous reactants as well as of solid reactants.
Liquids are generally much better ionic conductors at room temperature than most known solids, which is why liquids are normally used as the electrolyte of a room-temperature device. However, in some applications a solid electrolyte may be strongly preferred. For example, the Li-ion rechargeable battery uses a flammable organic liquid as the electrolyte, and a solid electrolyte would be safer and might be capable of improving the density of energy stored without sacrificing the rate of charge and discharge. Moreover, if the solid electrolyte also contains electric dipoles that give it a high dielectric constant, it can store much more electric energy than a liquid in an electric capacitance of an electric double layer of a metal/electrolyte interface.
In an electric-double-layer capacitor, metallic electrodes are fabricated so as to provide a maximum electrode/electrolyte interface. Ions in the electrolyte pin electrons or electron holes of opposite charge in the electrode across an electric double layer on charge. The separation of the electrons and holes across the double layer is small (atomic dimension) so the capacitance is large. On discharge, pinned electrons at the anode pass through the external circuit to recombine with the pinned electron holes in the cathode, and the mobile ions inside the electrolyte return to an equilibrium position. If the electrolyte has a large dielectric constant ∈′r, the capacitance of the electric double layer is enhanced. With a solid electrolyte having a large dielectric constant, the enhancement of the capacitance is large, and it becomes possible to construct a cell where the energy stored has a Faradaic component as in a battery and a capacitive component as in an electric-double-layer capacitor.